This project is a study of morphological changes that occur in the cochlea of aging gerbils. Some animals will be permanently housed in a very quiet environment. Others will be exposed to moderate levels of noise for extended periods. All animals will have their hearing levels monitored periodically throughout their lives so as to record changes as they occur. Terminal, acute physiological experiments will be done on each animal's cochlea and eighth nerve. The purpose of the morphological studies is to determine what structural changes accompany the degradation of cochlear function that is found during the terminal, acute physiology experiment. Cytocochleograms will be made to document hair cells loss. Because preliminary results have shown very little hair cell loss in quiet reared aged animals, special attention will be paid to the condition of the stria vascularis and spiral ligament. This will be done by cutting radial sections in the same tissue that was used to make the cytocochleograms. Cell densities and capillary densities will be measured in the stria vascularis and separately in the apical portion of the spiral ligament and in the ligament near the external sulcus. Preliminary results indicate that spiral ligament cells play a role in regulating ionic concentrations in cochlear fluids. For this reason the number and condition of these cells will be closely examined to see if they are affected when cochlear function is degraded. When necessary the radial cross sections will be examined by electron microscopy to better document changes that occur in aging cochleas.